Activated carbon is an excellent adsorbent. Activated carbon is commercially available in various forms, including granules and powders. In some applications of activated carbon, void spaces between carbon particles are important both for ensuring sufficient adsorbate contact and for allowing fluid to pass through the carbon adsorbent without encountering an excessive pressure drop. In other applications, however, the void space is not a principal concern. In the storage of hydrogen, ammonia, or natural gas, for example, carbon adsorption is most efficient on a volume basis when the carbon is formed into a high-density block with most of the void volume between the individual particles greatly reduced or even eliminated.
Efforts at creating such high-density solid structures are reflected in techniques developed for compaction and binding of activated carbon particles. For example, U.S. Pat. No. 4,000,236 to Redfarn et al. discloses a method for making a conglomerated activated carbon mass by means of a polymer rendered adhesive by a solvent. U.S. Pat. No. 4,717,595 to Watanabe et al. describes a method for producing carbonaceous material from carbon particles covered with a binder. U.S. Pat. No. 5,306,675 to Wu is directed to a method for producing activated carbon structures using methyl cellulose binders and microwave radiation curing.
These and other conventional techniques suffer from one or more serious drawbacks as follows: loss of surface area, corrupted pore distribution, limited temperature resistance, overly fragile green state, and high manufacturing costs. The carbon structures produced utilizing such methods also tend to have reduced surface area, lower adsorption capacity, and an undesirable pore size distribution. Furthermore, these carbon structures are not temperature-resistant, but tend to disintegrate when subjected to elevated temperatures.
Conventional binding techniques in particular cause a significant loss in available surface area for the activated carbon particles. With heretofore available techniques, binding and related agents are known to plug pores of the activated carbon particles whereby the favorable pore-size distribution of the original carbon particles is corrupted in favor of undesirably larger pore sizes.
This plugging phenomena is especially problematic for activated carbons with very high surface areas (&gt;2000 m.sup.2 /g). Efforts at using polymeric resins to bind very high surface area (&gt;2000 g/m.sup.2) carbons have generally failed heretofore because the resulting carbon structures had either surface areas greatly reduced from those of the original carbon particles or inadequate mechanical strength. In addition to reduced surface area, the thermal stability of carbon structures made by conventional techniques is inadequate for many otherwise appropriate applications.
Inorganic binders also have been used as binders in carbon mixtures to impart strength and thermal stability. For example, U.S. Pat. No. 4,518,704 to Okabayashi et al. describes a process for making activated carbon bodies using a clay binder. Unfortunately, a very expensive sintering step is required for such inorganic binders, e.g. firing at 900.degree. C. in an inert atmosphere. Furthermore, the mechanical strength of such bodies is inadequate for many applications.
Many uses for activated carbon require that the adsorbent fit into canisters and other devices of varying shapes and sizes. Such potential applications for activated carbon articles of manufacture thus far have gone unrealized because the required shapes and sizes for the solid articles could not be obtained. Standard binding methods for creating activated carbon structures often do not permit molding into unique shapes and sizes because the uncured, or green state, of the structure is either too fragile or too inflexible, thereby limiting workability.
Thus, there continues to be a need for improved very high surface area activated carbon structures as well as for methods for making such structures. The need also exists for methods of making activated carbon structures from activated carbon particles generally, without a substantial loss in carbon surface area.